Put the account file in the BOINC data directory (that varies by operating system)
Alternatively detach from your current account and attach using the weak key.

I did it from the command line (in linux) using: boinccmd --project_attach www.worldcommunitygrid.org <Jon's key>
For those who installed from Berkley you need to have the full path to the boinccmd binary, if from the repositories, that line will do. In Windows ... I don't know.